This article presents a cross-disciplinary approach to the study of constitutions: ‘constitutional institutionalism’. Conventional approaches in law, philosophy or political science tend to reduce constitutions either to their formal, factual or ideal aspects. The constitutional-institutionalist approach, by contrast, seeks to integrate these aspects into a more general perspective by focusing on the dynamic interplay between constitutional actors and constitutional norms. It understands constitutional norms as binding institutions that shape and constrain political action, but never fully determine it. Constitutional institutionalism furthermore asserts that constitutional norms, whatever form they take, only have meaning in relation to other constitutional norms as well as to constitutional actors, who impose meaning on these norms. Therefore, constitutional phenomena ultimately require interpretive explanations. This article concludes with a brief constitutional-institutionalist research agenda.